1. Field of the Invention
This invention relates to sheet feeding apparatus of the type by which single sheets may be successively fed from a stack of sheets, and is particularly directed to a sheet feeding apparatus including a movable platform for supporting the stack and the means for automatically moving the platform as successive sheets are fed from the stack so as to maintain a portion of the stack of sheets within a predetermined feed zone.
2. Description of the Prior Art
In processing machinery such as, by way of example, duplicating machines and the like, it is generally necessary to successively feed single sheets of material into the machine at a fairly high rate of speed while maintaining accurate registration of the sheets with the processing apparatus. In addition, it is desireable to store and successively feed a large quantity of sheets without requiring shutdown for reloading, thus maximizing the efficiency of the operation. To accomplish this, it is generally known to provide means for supporting a stack of sheets in or adjacent a feed zone and feed the sheets from the zone, one-at-a-time, into the apparatus. Generally, a movable platform or table is provided for supporting the stack and as the sheets are fed from the stack, the platform is advanced to move additional sheets into the feed zone.
In the duplicating art, for example, it is known to provide a mechanism for automatically raising a platform structure in order to maintain a portion of the stack of sheets at a predetermined level during feeding of the sheets into the machine. For example, as shown in U.S. Pat. No. 1,989,911, a platform adapted for supporting a stack of copy sheets is disposed such that the sheets are readily engaged by feed rollers and advanced into the duplicating machine. The platform rests on two pairs of levers with each pair being fixed to a rotatable shaft. The table is automatically raised in stages as sheets are fed from the sheet stack by the feed rollers by a ratchet segment which is attached to the shaft and is adapted to be engaged by a reciprocating pawl. When the reciprocating pawl engages the ratchet, it moves the ratchet segment and thereby raises the table. The reciprocating movement is given to the pawl by a rotatable cam fixed to a moving shaft of the duplicating machine. The pawl is normally biased out of engagement with the ratchet segment and is thrown into engagement with the ratchet segment only when the top sheet of the stack is below the predetermined feeding level. The device for throwing the pawl into engagement with the ratchet segment is controlled by the height of the feed roller as it cradles back and forth, depending upon the height of the top sheet in the stack. Over the years, cam driven raising mechanisms have been refined, as shown for example in U.S. Pat. No. 3,652,083. Additional cam driving mechanisms for sheet feeding apparatus are also illustrated in U.S. Pat. Nos. 2,204,715 and 3,883,132.
As operation of duplicating and copying machines became simplified and their use more commonplace, it became desireable to minimize the sheet handling mechanisms. For example, many copy machines are now equipped with spring-loaded platforms or cassettes for holding a quantity of sheets, the spring force continuously urging the uppermost sheet into engagement with the feed rollers of the machine. A problem with this type of device is caused by the spring force system, wherein the actual force with which the sheets are urged into engagement with the feed rollers varies depending upon the weight of the stack and the working length of the spring. Because of this, the quantity of sheets that can be stored and fed with this type of device is necessarily limited.